1. Field of the Invention
The present invention relates to a dielectric film configuration of a semiconductor device, and more particularly to a semiconductor device having a dielectric film formed by performing thermal oxidation on the surface of a single crystalline substrate.
2. Description of the Background Art
A semiconductor device having an insulated-gate type device such as MOSFET and IGBT driven by using field effect includes a gate dielectric film formed on a single crystalline substrate such as silicon and a gate electrode formed on the gate dielectric film. The gate dielectric film exerts a great influence upon the performance of such semiconductor device. For achieving stabilized operations of such device, it is preferable that the gate dielectric film should have a great resistance to an electric field (i.e., have a high dielectric withstand voltage) and should be thin. Therefore, a thermal oxide film of uniform properties and thickness is commonly used as such gate dielectric film.
For instance, nonuniformity of such gate dielectric film in thickness with even one portion being extremely thin will lower the dielectric withstand voltage of the entire gate dielectric film. Particularly in recent years, this problem is becoming more serious as gate dielectric films are becoming increasingly thinner.
There is a technique of removing irregularities of configuration of a gate dielectric film for increasing the dielectric withstand voltage (e.g., Japanese Patent Application Laid-Open No. 1-138760 (1989); pp.2-3, FIG. 1). 
A thermal oxide film used as the gate dielectric film is formed thin on the surface of a single crystalline substrate by creating a chemical bond between the surface of the single crystalline substrate and oxygen by heat treatment in an oxidizing environment. That is, the thermal oxide film is generated by a chemical bond between oxygen and a substrate material (e.g., silicon), and is thus not formed in a region unexposed to the outside of the single crystalline substrate. Further, the thermal oxide film is difficult to form in a region where some shield interferes with a sufficient supply of oxygen or a substrate material to be bonded to oxygen.
Therefore, when another dielectric film formed in a previous step, for example, exists in a region where the thermal oxide film is to be formed, it is difficult to supply a substrate material necessary for thermal oxidation in the vicinity of such dielectric film formed in the previous step, causing a boundary portion of the thermal oxide film with respect to the dielectric film to be locally reduced in film thickness. Through the use of such thermal oxide film as the gate dielectric film, a high gate dielectric withstand voltage is not available, preventing stabilized operations of a semiconductor device, which results in degraded operational reliability.
For instance, in a manufacturing step of a power MOSFET which is one of insulated-gate type devices, a dielectric film for maintaining a withstand voltage at an edge part of a semiconductor chip is formed on a single crystalline silicon substrate, and thereafter, a gate dielectric film in contact thereto is formed by thermal oxidation. This causes a boundary portion of the gate dielectric film with respect to the dielectric film previously formed to be locally reduced in film thickness, resulting in the above-described problem.